This invention relates to electronic circuits, and more particularly to an electronic variable gain amplifier with high linearity and low noise.
In a radio frequency (RF) transceiver, the received signal has a high dynamic range ( greater than 80 dB). In order to supply a signal of constant amplitude to a baseband section of the transceiver, a variable gain amplifier (VGA) with equivalent or better dynamic range is required. It is desirable that the VGA exhibit low noise when a small signal is present at the input in a high gain mode, and yet be able to handle a large input signal when in a low gain mode. Accordingly, the VGA should exhibit high linearity in low gain mode. Known solutions fail to provide a VGA that has sufficiently low noise, high linearity, linear gain control, temperature stable gain control, and a high dynamic range that are all suitable for some applications (such as a code division multiple access (CDMA) receiver).
Accordingly, the inventor has perceived that there is a need for a variable gain amplifier which has such characteristics. The present invention provides such an amplifier.
The invention includes a variable gain amplifier that has a high linearity gain stage coupled with a low noise gain stage, and a steering circuit that steers the input signal between these two stages. The preferred embodiment employs steering of the input signal and extensive linearization (in dB) of the control signal to provide stable and linear gain control and high dynamic range.
In particular, in one aspect the invention includes a first differential amplifier stage having low noise (i.e., no emitter degeneration in a bipolar implementation) and configured to receive a differential input signal and contribute to an output signal current; a second differential amplifier stage that has higher linearity than the first differential amplifier stage (but has more noise, i.e., exhibits emitter degeneration in a bipolar implementation), and configured to receive the differential input signal and contribute to the output signal current; a first current source coupled to the first differential amplifier stage and regulated by a control signal x according to the formula I*(kxe2x88x92x), where k is a constant and x is a variable ranging from 0 to k; a second current source coupled to the second differential amplifier stage and regulated by a control signal x according to the formula I*(x); and a signal steering circuit, coupled to the first and second differential amplifier stages, for steering the output signal current from the first and second differential amplifier stages either to a voltage source or to a corresponding load.
The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims.